The invention concerns providing a fuse in a solid electrolyte capacitor such as a tantalum capacitor.
With the particular objective of reducing the deleterious consequences of a short-circuit in a circuit including one or more solid electrolyte capacitors, attempts have been made to incorporate a fuse into the capacitor while minimizing the resulting increase in overall dimensions.
A solid electrolyte capacitor, of the tantalum type in particular, principally comprises a porous anode from which projects an anode wire and which is covered partially or completely with various conventional layers of dielectric/oxide, manganese dioxide substantially filling the pores of the anode and constituting the solid elctrolyte, and a conductive layer forming the counter-electrode or cathode. This constitutes a capacitor body that is then surrounded with an electrically insulative material after fixing to the electrodes connecting leads ending in output terminals. Extremely compact capacitors of simple geometrical shape (cylinder or usually rectangular parallelepiped) are obtained this way.
Various capacitors of this type with integral fuse are known already.
For example, U.S. Pat. No. 4,107,762 describes a capacitor in which the lead forming the negative terminal is connected to the counter-electrode by a fuse embedded in the insulative coating, this fuse being formed by a twin tape of aluminum and palladium. In the event of a short-circuit the exothermic heating of this fuse results in the alloying of these elements and an electrical break between this negative terminal and the counter-electrode.
This solution, which is satisfactory in many cases, nevertheless has the disadvantage of relatively wide dispersion in the effective or useful length of these fuses and therefore in the associated current thresholds.
To alleviate this disadvantage it has subsequently been proposed in U.S. Pat. No. 4,224,656 to use a fusible block comprising a parallel sided insulative block through which extend a hole and a fusible wire parallel to the axis of the hole, the effective length of the wire therefore being determined in a very precise way.
The accuracy of this fusible block is improved in that this effective length of the fusible wire is in a thermally insulative air pocket which prevents the heat generated in the wire by the JOULE effect diffusing into the capacitor and slowing down the breaking of the circuit. The fabrication of such blocks is always relatively long and delicate.
To facilitate manufacture with a comparable level of performance U.S. Pat. No. 4,720,772 proposes a fusible member comprising an insulating plate on which two separate conductive layers of very precise geometry are connected by a fusible wire disposed at a determined location by virtue of which its effective length is precisely determined. This fusible wire is coated with a thermally insulative elastic material such as a silicone resin in which bubbles are formed for improved thermal insulation.
It will be noted that patents U.S. Pat. Nos. 4,224,656 and 4,720,772 achieve "calibration" of the effective length the fusible wire through the use of an additional support member apparently necessary during manufacture to maintain a predetermined distance between the areas in which the ends of the fusible wire must be fixed.
An object of the invention is to further simplify the integration of a fuse into a solid electrolyte capacitor and therefore to reduce the cost thereof, while ensuring a comparable level of performance, through eliminating any intermediate support member attached between the negative connecting lead and the counter-electrode.
To this end it proposes to place a predetermined effective length of calibrated fusible wire between two electrically insulated sections of this lead, which is directly connected by one of its sections to the counterelectrode.